I've never seen it as a smile. I always thought he was just yelling. And while it is true that he hates fighting, it doesn't mean he won't DO it. He is yelling because he just overcame a powerful opponent. The biggest lie the fandom tells about X is that he is a weak, whimpering little baby who is too emotionally fragile to handle the horrors of war. The exact opposite is true. He's like Kenshiro from Fist of the North Star. He has all the power to obliterate any opponent, but all he really wants is to live a peaceful life. Aversion to conflict and pacifism is not part and parcel to being fragile and limp. Those two things often coincide, but they are not the same and can exist separately.